Bascular bridges are often used as drawbridges for pedestrian, vehicular, and railroad traffic. A bascular design for a drawbridge is advantageous since bridges typically comprise large, heavy structural components which are difficult to lift. A counterbalanced bascular bridge makes lifting the bridge deck easier. A large amount of energy is required just to star the heavy bridge moving without having to lift all of the bridge's weight. However, by counterbalancing the bridge, weight is necessarily added to the entire structure, making the whole a very cumbersome device to move quickly. Thus traffic is typically tied up waiting long periods for the bridge to open sufficiently for boats to pass and then to slowly close again.
Weight is also added to the typical drawbridge due to its cantilevered construction. The two sides of the bridge project inwardly and are essentially two simple cantilevered beams. Heavy loads at the end of a simple cantilevered beam (the location corresponding to the center of the bridge) are difficult to carry and, as a result, the bridge must have a heavy duty construction.
Attempts at solving this problem have taken two approaches. The first approach is centered on connecting the two bridge sections in the middle so that they operate as a single rigid span. The patent to Vent (U.S. Pat. No. 683,627) is an example of such a bridge. I The problems with such a construction include a complicated interconnect arrangement due to the fact that the bridge sections are pivoting down into place. The complexity may affect the strength of the interconnection, the cost of construction, and the speed of connection. Added weight that must then be lifted when opening the bridge is also a result of this construction. This added weight is necessarily in the location most difficult to lift.
The second approach involves the two bridge sections meeting at their ends and bearing against each other at a point sufficiently above their pivotal or rolling supports to cause them to operate as an arch construction. The patent to Adams et al. (U.S. Pat. No. 173,253) is an example of this basic construction. The arch construction imposes oblique loads or stresses upon the foundations, causing them to settle unevenly and become distorted in shape. It also poses alignment problems as the proper mating of the ends of the section is critical to the proper distribution of the loads. Another obvious problem is the resultant arched road surface. This may be overcome by appropriate build up of the sections, but such build up adds weight.
Other problems arise with typical drawbridges due to their span. For example, with the bridge in an open position high winds exert tremendous forces on the bridge deck. To protect against failure, added structural support to account for these forces must be provided.
Attempts to solve the problems associated with the long span have been made. For example, a patent to Worden (U.S. Pat. No. 534,704) discloses a drawbridge with two hingeably connected bridge leaves on each side that fold together when retracted. However, this structure may still be difficult to use as it employs no counter weight. It must also utilize heavy structural members since once closed and ready for traffic it is essentially two cantilevered beams. Supporting weight in the center of the bridge is more difficult with this arrangement.
In consideration of the limitations of the devices disclosed in the prior art discussed above, it should be apparent that an effective solution to the problem of quickly and efficiently reversibly spanning a distance with a bridge is needed. Accordingly, the present invention was developed and provides significant advantages over previous bridges used to reversibly span over waterways and other obstacles.